JP3021526B2 - Electrolyte for lithium secondary battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrolyte for a lithium secondary battery.

_2. Description of the Related Art Conventionally, a lithium secondary battery uses a transition metal such as molybdenum disulfide (MoS ₂ ), titanium disulfide (TiS ₂ ), manganese dioxide (MnO ₂ ), or vanadium pentoxide (V ₂ O ₅ ) as a positive electrode active material. A battery system using a sulfide or an oxide and using an alloy that absorbs and releases metallic lithium and lithium ions in the negative electrode, such as a wood alloy and a lithium aluminum alloy, is known.

With respect to the charge / discharge characteristics of the positive electrode, the utilization rate decreases from the initial stage, stabilizes from a certain number of cycles, and the charge / discharge efficiency (discharge capacity / charge capacity × 100) is very high, nearly 100%. However, with regard to the negative electrode, the charge and discharge efficiency is up to 98% for lithium metal and 99% for an alloy that absorbs and releases lithium ions. Therefore, when the positive / negative electrode capacity ratio is 1: 1 and the negative electrode capacity of the battery decreases by 1% to 2% for each cycle, the cycle life of the battery is 25% to 50% of the initial capacity.
50 cycles. As is clear from these, it is understood that the negative electrode controls the cycle life of the lithium secondary battery.

Problems to be Solved by the Invention The difference between the charge and discharge efficiencies of the positive electrode and the negative electrode generally arises from the fact that the organic solvent is strong in oxidation, ie, a reaction in which electrons are extracted from molecules, and is weak in reduction. For example, the oxidative decomposition potential is as high as 5 V or more with respect to lithium. For example, gamma-butyrolactone (GBL) used for lithium fluoride graphite primary batteries undergoes reductive decomposition when charging lithium secondary batteries, and the charge / discharge efficiency of the negative electrode is low. Become. In particular, when lithium metal is used for the negative electrode, the charge potential is lower than the reduction potential of the solvent, and the charge / discharge efficiency is lower than that of an alloy that stores and releases lithium ions.

When the negative electrode is an alloy that stores and releases lithium ions, unlike when charging lithium metal, lithium is stored in the alloy and needle-like crystals are not generated, so that the charge and discharge efficiency is higher than that of lithium metal. However, the capacity density is 2062mAh / cc for lithium metal, but 1700mAh / cc for alloys, and the discharge potential is 0.2-0.6V noble than lithium metal, so the energy density is low in batteries with limited internal volume. Become.

Therefore, in order to realize a lithium secondary battery with a high energy density, it is considered better to use lithium metal as the negative electrode. However, the charge / discharge efficiency is as low as about 98%. For example, in order to secure a battery life of 300 cycles like a lead-acid battery, lithium is about 6 times the charge / discharge capacity of the positive electrode. It is a situation that has to be low from a viewpoint.

As described above, it is difficult to achieve a high energy density and a long cycle life with a conventional battery, and an electrolyte for a lithium secondary battery having excellent reduction resistance is required.

An object of the present invention is to solve such a problem.

Means for Solving the Problems In order to solve the above problems, the present invention provides a gamma alkoxybutyrolactone represented by the following structural formula in an electrolyte for a lithium secondary battery. (RO—alkoxy group having 1 to 4 carbon atoms) alone or as a mixed solvent with another solvent.

Effect By using the above-mentioned electrolytic solution, the solvent is hardly reduced at the time of charging, so that the charge and discharge efficiency of the negative electrode is improved, and a lithium secondary battery having a long cycle life and a high energy density can be obtained.

Embodiment An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a partial sectional view of a lithium secondary battery according to the present invention, for example, a battery having a diameter of 20 mm and a height of 1.6 mm.

In FIG. 1, reference numeral 1 denotes a polypropylene separator impregnated with an electrolytic solution obtained by dissolving 1 mol / mol of lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte in an organic solvent (hereinafter referred to as a solvent). Gamma alkoxybutyrolactone represented by the following structural formula as a main solvent of this solvent (RO- is an alkoxy group having 1 to 4 carbon atoms).

Reference numeral 2 denotes lithium metal as a negative electrode active material, which is fixed by pressure to a stainless steel negative electrode current collector 4 formed on the inner surface of a stainless steel sealing plate 3. Reference numeral 5 denotes a positive electrode mixture using manganese dioxide as a positive electrode active material, which is fixed by pressure to a positive electrode collector 7 made of titanium and fixed to the inner surface of a case 6 made of stainless steel. 8 is a polypropylene gasket. For example, the positive electrode mixture 5 has a composition of 100 parts by weight of manganese dioxide, carbon black 20, and a fluororesin binder 20,
The capacity is set to be 10 mAh. Lithium metal 2
Has a capacity of 25 mAh.

Here, FIG. 2 shows the results of examining the cycle characteristics of the battery having the above-described structure when the solvent was varied. Note that charging was performed up to 3.8 V with a current of 0.5 mA, and discharging was performed up to 2.0 V with a current of 1.5 mA. (A) to (G) in FIG.
Table 1 shows the components of the solvent up to. The concentration of LIPF _{6 in} the electrolyte is 1 mol /.

For comparison, gamma-butyrolactone (GBL),
Gamma valerolactone (GVL) and propylene carbonate (PC) are also shown.

FIG. 2 shows that the batteries using the solvent of the present invention ((B) and (C) in the figure) have a longer cycle life than the batteries using other solvents.

GVL has a methyl group attached to the γ-position of GBL as a side chain, and has strong oxidation resistance. However, it does not contribute to reduction resistance.

In contrast to these solvents, the solvent of the present invention has an alkoxy group that contributes to reduction resistance, has sufficient reduction resistance, and also retains high oxidation resistance. Is very useful.

However, when the alkoxy group has more than 4 carbon atoms,
Molecules themselves are bulky structurally, causing an increase in viscosity and making it difficult to use as a solvent for a lithium secondary battery.

Result calculated by the following equation charging and discharging efficiency of the negative electrode from cycle life obtained from Figure 2, the average charge-discharge efficiency;. Eff (%) = ( Q-Q ex / n) Qx100 where, Q is the cell Average charge / discharge capacity (mAh), Q _ex is capacity (mAh) excluding lithium capacity remaining in the positive electrode during charge / discharge from lithium capacity, and n is cycle life, where capacity degraded by 50% of initial capacity The number of cycles at the time was defined as the cycle life.

The average charge and discharge efficiency of the electrolytic solution within the scope of the present invention is all 9
From 8.5 to 99.0%, it can be seen that the value is higher than that of the battery using the conventional electrolytic solution. Accordingly, the capacity ratio of the positive electrode to the negative electrode for obtaining a cycle life of 300 cycles from the average charge and discharge efficiency of the electrolyte according to the present invention is 3 to 4.5 times the charge and discharge capacity of the positive electrode. Compared with the conventional method, the volumetric efficiency of the battery is improved, and if the capacity of the negative electrode is further reduced, the capacity of the positive electrode can be increased by the reduced volume, so that a lithium secondary battery having a higher energy density than before can be obtained. In addition, the charge-discharge efficiency of the negative electrode and the reduction resistance of the solvent are closely related, and the amount of lithium consumed by the reaction between the electrolytic solution and lithium during charging decreases as the solvent having higher resistance to reduction is used. The charge and discharge efficiency of the negative electrode is increased. Solvents having high resistance to reduction include ethylene carbonate, propylene carbonate, tetrahydrofuran, 1,2-dimethoxyethane, acetonitrile, dimethylformamide, and the like, which are conventionally known, and are appropriately combined with these and mixed. Even in this state, the charge / discharge efficiency of the negative electrode can be increased and the charge / discharge cycle life characteristics of the battery can be improved. For this reason, when the amount of lithium is the same, the charge and discharge efficiency of the negative electrode becomes higher and the cycle life becomes longer as the solvent having higher reduction resistance is used. Therefore, it can be said that the solvent of the present invention has high reduction resistance and is excellent as a solvent used for an electrolyte for a lithium secondary battery.

Effects of the Invention As is apparent from the above description, according to the present invention, a lithium secondary battery having characteristics of a higher energy density and a longer cycle life than conventional ones can be obtained.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view of a lithium secondary battery according to one embodiment of the present invention, and FIG. 2 is a diagram showing cycle characteristics of the lithium secondary battery. 1 ... separator, 2 ... lithium metal, 4 ... positive electrode mixture.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-57-152683 (JP, A) JP-A-61-32961 (JP, A) JP-A-62-290073 (JP, A) JP-A-63-62973 32871 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 10/40 H01M 6/16

Claims (1)

(57) [Claims] An electrolyte for a lithium secondary battery using a solvent in which a lithium salt is dissolved as an electrolyte, wherein the solvent is a gamma-alkoxybutyrolactone represented by the following structural formula: (RO- is an alkoxy group having 1 to 4 carbon atoms) alone or in a mixed solvent with another solvent.